Biological classification keeps changing with our understanding of life forms. Initially, new characters were taken by Aristotle, Theophrastus, John Ray and Linnaeus. Linnaeus divided plants on the basis of sex organs only.
As more and more knowledge gathered for delimitation of taxa, the ways for natural system of classification became clear.
This can be understood by following examples
(i) Two kingdom classification was replaced by three kingdom classification when Haeckel (1866) proposed the kingdom of Protista.
(ii) It Was converted into four kingdom classification by Copeland when he proposed the kingdom-Monera.
(iii) Whittaker in 1969 proposed five kingdom classification.
(iv) As the information about biochemistry of archaebacteria gathered, Woese raised another
kingdom, Archaea.